This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 10-339685, filed Nov. 30, 1998; and No. 11-328865, filed Nov. 18, 1999, the entire contents of both of which are incorporated herein by reference.
The present invention relates to a tensioner for appropriately maintaining the tension of a force transmitting member, such as an endless belt or endless chain, in a power transmission mechanism that uses the force transmitting member.
A force transmitting member, such as an endless belt or chain, is used in a power transmission mechanism that transmits rotary motion to cam shaft in an engine of an automobile, for example. In some cases, a tensioner is used to keep the tension of the force transmitting member appropriate. FIGS. 21 and 22 individually show sections of a conventional tensioner. This tensioner is provided with a casing 1. A first shaft member 2 and a tubular second shaft member 3 are inserted in the casing 1. The casing 1 is provided with a flange portion 1b that has a mounting hole 1a for fixation on an apparatus such as an engine. An external thread portion is formed on the outer surface of the first shaft member 2. An internal thread portion is formed on the inner surface of the second shaft member 3. These external and internal thread portions mate with each other. A rear end portion 2a of the first shaft member 2 is inserted in a fitting hole 9 that is formed inside the casing 1. The end face of the rear end portion 2a is in contact with the inner surface of the casing 1. A torsion spring 4 is provided around the first shaft member 2. One end 4a of the torsion spring 4 is anchored to the first shaft member 2, while the other end 4b is anchored to the casing 1. If the spring 4 is twisted, the repulsive force of the spring 4 generates torque that causes the first shaft member 2 to rotate. The first shaft member 2 is rotatable with respect to the casing 1.
The cylindrical second shaft member 3 penetrates a sliding hole 5a that is formed in a bearing 5. As shown in FIG. 22, both the outer peripheral surface of the second shaft member 3 and the inner peripheral surface of the sliding hole 5a are noncircular. Thus, the second shaft member 3 is allowed to move in its axial direction with respect to the bearing 5, and is prevented from rotating. If the first shaft member 2 is rotated by means of the repulsive force of the torsion spring 4, therefore, the second shaft member 3 generates an axial thrust without rotating. For example, the repulsive force of the spring 4 acts in a direction such that it causes the second shaft member 3 to project from the casing 1. A moderate tension can be applied to the aforesaid force transmitting member, the belt or chain, by applying this thrust to the force transmitting member. If the second shaft member 3 pushes the force transmitting member, a reactive force from the force transmitting member acts on the shaft member 3. The shaft member 3 moves in its axial direction to a position such that this reaction force (input load) balances with the thrust of the shaft member 3 that is generated by means of the torsion spring 4. Thus, the conventional tensioner has a linear characteristic such that the input load is proportional to the movement of the second shaft member 3.
The tension of the force transmitting member, the chain or belt, continuously changes depending on the operating conditions of the engine, for example. Since the conventional tensioner has linear characteristics, however, it cannot easily cope with a wide variation in input load.
The following is a description of the relation between the force (thrust) of the tensioner which pushes the force transmitting member and a displacement amplitude "sgr" of the tensioner. The stiffness of the tensioner can be represented by the movement (i.e., displacement amplitude "sgr") of the second shaft member relative to the load received from the force transmitting member. Although a tensioner with great thrust and high stiffness can resist a heavy input load, its displacement amplitude a is small. If the thrust of the tensioner is made smaller, in contrast with this, a heavy input load cannot be coped with, although the displacement amplitude "sgr" can be made greater. The displacement amplitude "sgr" becomes smaller if the stiffness of the tensioner for a large engine displacement is enhanced. Thus, a high-stiffness tensioner must inevitably be designed to function within a narrow range of displacement amplitude "sgr", that is, the degree of freedom of the tensioner design is low.
The object of the present invention is to provide a tensioner capable of coping with a large variation of amplitude despite its high stiffness, thereby dealing with a wide range of input loads.
A tensioner of the present invention comprises: a first shaft member rotatably inserted in a casing so as to be restrained from axial movement and having a first thread portion; a second shaft member having a second thread portion mating with the first thread portion, axially movable with respect to the casing, and restrained from rotation; a torsion spring for generating torque capable of rotating the first shaft member; and torque switching means for changing the turning torque of the first shaft member in accordance with the rotational angle of the first shaft member.
The torque switching means can use a torque switching member that is adapted to generate a small frictional torque when the rotational angle of the first shaft member is narrow and to generate a large frictional torque when the rotational angle is wide.
In the tensioner of this invention, the load applied to the second shaft from a force transmitting member such as a belt or chain causes the first thread portion and the second thread portion to rotate the first shaft member. As long as the rotational angle after the start of rotation of the first shaft member is narrow, the torque switching member generates a small turning torque. If the rotational angle of the first shaft member becomes wider, the torque switching member generates a strong turning torque. Thus, a heavy received load can be coped with, and a small amplitude displacement can be followed satisfactorily. The force transmitting member that is used in a large-displacement engine or the like, for example, can cope with wide variations in the received load, and an appropriate tension can be applied to the force transmitting member.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.